1. Field of the Invention
This invention relates to an automatic temperature verification device and more particularly to a device which provides a visual indication of an ambient temperature above or below a first predetermined temperature level and further provides for maintaining a permanent visible indication in the event the ambient temperature passes above or below a second predetermined temperature level.
2. Statement of the Prior Art
Frequently in the use, transporation, and storage of materials at critical temperatures, failures occur in the power system or elsewhere which allow the ambient temperature to rise above or fall below a predetermined level. There is a need for an inexpensive temperature indicator device which will provide a visible indication of the transition of the storage temperature above or below a first critical temperature level. Appearance of the visible indication permits the user to take corrective steps before permanent deterioration of the goods. As an example, in the storage of frozen foods, there is a need for the operator to be aware if the storage temperature rises above a first critical level. The operator can take corrective measures to prevent further temperature rises, for example, by promptly transferring the foods to another storage container where the adequate low temperatures are maintained.
There is a further need for the operator to know if the temperature has at any time passed a critical temperature level at which deterioration occurs. For example, in the storage of frozen foods, if a power outage occurs while no person is in attendance, the frozen food might rise above required temperature levels and thereafter, upon restoration of power, the storage temperature would be returned to an acceptable frozen food storage temperature. Irreversible damage to the thawed food occurs under these circumstances and the operator is not aware that the damage has occurred. Accordingly there is a need for a visible indicator device which will provide a notice that the temperature has passed a second critical temperature level, despite the fact that the ambient temperature in the compartment has returned to an acceptable level.
There is a further need for a temperature verification device that is not rapid in its action. A slow, but affirmative response temperature indicator is required for storage areas where doors are open for short periods of time to load or remove material. Some inherent delay in response of the temperature verification device is desirable. The temperature verification device furthermore should be reasonably tamperproof so that thin wires or small tools cannot be inserted to compromise the device after its set point has been established.
There is a further need for an inexpensive device which can accommodate both described functions and further can accommodate either temperature which exceeds a maximum, or temperature which falls below a minimum.
The copending U.S. patent application Ser. No. 173,054, filed July 28, 1980, discloses a useful temperature indicating device which inexpensively and compactly accomplishes the stated objectives of the present invention. In that copending patent application, the outer end of a coiled temperature-responsive bimetallic strip rotates and establishes a predetermined position relative to the casing in accordance with the ambient temperature so long as a predetermined maximum or minimum temperature is not exceeded. The outer end of the coiled bimetallic strip is secured to a temperature indicating element which moves corresponding to the outer end of the coiled bimetallic strip. When the predetermined maximum or minimum temperature is exceeded, a locking device is released to prevent reverse movement of the temperature indicating element and the outer end of the bimetallic strip. The assembly of that device requires positioning of a number of individual components. The device furthermore requires a somewhat delicate factory setting.